feat(os): port Ghostty's Linux OS helpers

Ports ZIG/src/os for Linux into ghostty-os: XDG dirs, home dir, hostname,
env/PATH helpers, passwd lookup (via nix), shell detection, resources
dir discovery, string encodings (printf-q, URL percent), systemd
sd_notify, i18n via gettextrs, xdg-open, desktop/cgroup/kernel info,
temp dirs, URI parsing (port of std.Uri incl. Ghostty's extra options),
and a zbus-based Flatpak HostCommand implementation.

The crate stays free of unsafe. Consequences, all tracked as
TODO(integration) in docs/rewrite/notes/ghostty-os.md:
- locale::ensure_locale is a no-op (setlocale/setenv are unsafe; GTK
  calls setlocale itself); setenv/unsetenv are not exposed.
- passwd's Flatpak branch needs the termio pty and falls back to the
  sandbox entry for now.
- FlatpakHostCommand's D-Bus path is untested (no portal here).
Intentional fix: a leading '@' in NOTIFY_SOCKET is treated as an
abstract socket.

macOS/Windows/WASM modules are out of scope.

Test Plan:
- cargo test -p ghostty-os (88 tests)
- cargo clippy -p ghostty-os --all-targets -- -D warnings

crates/ghostty-os/Cargo.toml

@@ -5,7 +5,16 @@ edition.workspace = true
 license.workspace = true
 
 [dependencies]
+gettext-rs = { version = "0.8.0", features = ["gettext-system"] }
+log = "0.4.34"
+nix = { version = "0.31.3", features = ["user"] }
+rustix = { version = "1.1.5", features = ["fs", "process", "system", "pipe", "net", "time", "rand"] }
+thiserror = "2.0.21"
+zbus = "5.19.0"
 
 
 [lints]
 workspace = true
+
+[dev-dependencies]
+temp-env = "0.3.6"